1. Field of the Invention
This invention relates to liquid crystal compositions which include a specific biphenyl-like liquid crystal material and the method for making the same.
2. Prior Art
The use of liquid crystal materials in timepieces and the like are well-known in the art. Displays including these liquid crystal materials are often referred to as liquid-crystal displays (LCDs) which are not self illuminating; they simply absorb or scatter ambient light and thus operate either as a reflective or a transmissive mode. The LCD is turned on and made visible by a voltage-controlled change in the refractive index of the display medium. Nematic liquid-crystals presently dominate the liquid-crystal display technology. The word "nematic" is derived from the Greek word for thread and refers to microscopic thread-like boundaries separating the LC medium's various domains of molecular orientation. The term "liquid crystal" refers to the nature of the display medium; it is organic, and pours, flows and otherwise behaves as a normal liquid, yet its cigar-shaped molecules have an orderliness that makes the fluid behave optically like a crystal.
Apart from the nematic, the cholesteric type of liquid crystals is one the few other LC phases of current interest in display technology. Cholesteric liquid crystals change color under the influence of temperature and pressure, for example, but mixtures of these liquid crystals have had problems in terms of their storage stability properties. In a typical display device, the basic display consists of a thin layer of LC solution sandwiched between two thin glass plates. A transparent electrode material, usually tin oxide, forms a 7-segment numeral on the inside surface of the front plate, as well as a non-segmented electrode pattern on the inside surface of the rear plate. In reflective structures, a highly-reflective electrode material, such as chromium, aluminum or nickel, which serves as a mirror, is deposited on the rear plate.
In its quiescent state, the LC medium is transparent, (i.e., the display is dark) because of the orderly array of its cigar-shape molecules. When a voltage is applied between the electrodes, an electric field is created that disorders the LC molecules, thus rendering the medium opaque in the vicinity of the field, scattering the light. The segmented electrodes selectively activate the medium; activated areas appear brighter than the unactivated areas, and a numeral appears. This type of operation is generally referred to in the art as dynamic scattering. Because light is scattered away (in the same direction as that of the incident light), dynamic scattering operation requires a reflector when the observer's eye is in front of the LC cell, as in a watch display. Direct-current operation shortens life, therefore, such display use an AC drive signal.
Field-effect LCD's transmit polarized light in their quiescent state and block it in the presence of an electric field. Current flow is not necessary, as in the dynamic-scattering mode. In a field-effect LCD, the preferred alignment of molecules near one glass plate is at 90 degrees to the alignment near the other plate. In effect, the LC molecule alignment is twisted through 90 degrees across the thickness of the cell. Thus, field-effect operation is also referred to as twisted-nematic operation.
Although the basic cell structure is the same as that of the dynamic-scattering cell, polarizer elements are added, crossed at 90 degrees, at the front and rear plates. Polarized light enters the unactivated cell, is rotated 90 degrees by the molecular twist of the LC medium, and thus exits the cell through the cross-polarizer of the rear. When activated by an electric field, the molecular alignment "straightens-out", and passes the polarized incident light directly to the rear cross-polarizer without further rotation. The rear polarizer now blocks the light, and an observer behind the cell sees a dark display on a light background. Angularly aligning the polarizers creates a light display on a dark background. Placing a mirror behind the second polarizer creates a reflective cell.
Reflective field effect LCD's provide a significantly higher contrast ratio and thus are generally easier to read than reflective dynamic-scattering devices. It is also possible to generate color in field effect displays; however, the same problem is present, namely, the use of the liquid crystal material therein to create such color and light scattering is not thermally or chemically stable.
Thus, the above-identified problems are of significant proportions given the fact that chemical contamination, including impurities from the glass, and discoloration, can render the entire display device useless. Shortened lifetime and poor performance results from contaminated LC material and is further enhanced because the rate of decomposition is accelerated when the device is turned on.
Examples of electrochromatic displays are disclosed in U.S. Pat. No. 3,839,857 and U.S. Pat. No. 3,652,149.
When one traces the history of the materials used in such display elements, a variety of liquid crystal materials have been developed and introduced. Among those materials used include biphenyl liquid crystals and ester liquid crystal. These type of liquid crystals have been used because of their relatively good colorlessness and chemical stability. However, these prior art materials, by themselves, have a rather narrow temperature stability range and therefore are generally used as a mixed composition with other more temperature stable materials. For example, the operating temperature range of typical biphenyl liquid crystal material is as follows:
______________________________________ Liquid Crystal Operating Temperature Range ______________________________________ ##STR2## 22.degree.-35.degree. C. ##STR3## 58.degree.-76.degree. C. Equal mixture by mole of (1) and (2) 10.degree.-55.degree. C. ______________________________________
In the case of ester based liquid crystals
______________________________________ ##STR4## -42.degree. C. ##STR5## 45.degree.-72.degree. C. ##STR6## 45.degree.-70.degree. C. ______________________________________
As described above, these conventional biphenyl and ester liquid crystal materials have a generally narrow temperature range for displaying purposes. The lowest point of the temperature range indicates the transient temperature from solid state to nematic state and the highest point indicates the transient temperature from nematic state to isotropic state.
The purpose of this invention is to provide a liquid crystal material which has excellent chemical stability, at least as good as the biphenyl liquid crystal materials and estel liquid crystal materials and, in addition, has a broad range of temperature stability. In order to achieve this object, biphenyl benzoyl ester having a lateral chlorine substitution is derived.
The novel features which are believed to be characteristic of the invention, both as its organization and method of operation, together with further objectives and advantages thereof, will be better understood from the following description in which presently preferred embodiments of the invention are illustrated by way of examples. It is to be expressly understood, however, that the examples are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.